The role of chromosomal proteins in maintaining the structure and regulating the function of chromatin and chromosomes is studied. Antibodies against a specific fraction of chromosomal nonhistone proteins have been elicited and the major antigenic entities identified. The binding of monoclonal antibodies to histone H5 and H1 has been mapped in detail. Chromatin fragments highly enriched in HMG-17 and H1 have been purified by immunoaffinity chromatography. The exchange of chromosomal proteins among various chromatin fragments has been studied. Details of the interactions of chromosomal proteins HMG-1 and HMG-2 with various DNA structures have been elucidated. The results indicate that chromosomal proteins HMG-1 and HMG-2 are single-stranded DNA binding proteins which can distinguish between similar DNA structures. These proteins constitute a component that is obligatory for proper cell function. They serve as structural elements maintaining specific chromatin regions in a particular conformation which may be recognizable by regulatory elements.